Hydrophone



Dec. 8l 1925- HYDROPHONE Filed may 16. 1921 2 Sheets-Sheet 2 Me/TURB ,c1/JMJ Mai/U /77- TDR/VEz/E r/ N@ am U/ ww @m M E (Il y j Si ...10.0 www1 4 WALTER H. BARBER, OF STONEHAM, AND RICHARD D. FAY, OF NAIIANT, MASSA- CHUSETTS, ASSIGNORS TO SUBMARINE SIGNAL COMPANY, OF BOSTON, MASSA- CHUSETTS, A CORPORATION OF MAINE.

HYDROPHONE.

Application led May 16,

To all who/m. t may concern:

Be it known that we, WALTER H. BARBER, of Stoneham, in the county of Middlesex and State of Massachusetts, and RICHARD D. FAY, of Nahant, in the county of Essex, in said State, both citizens of the United States, have invented a new and useful Imrovement in Hydrophones, of which the ollowing is a specification.

We have made a new and useful improvement in subaqueous sound receiving devices which are today known as hydrophones. Our invention relates particularly to that class of hydrophone wh1ch is used primarily to detect the sounds produced by a source having a definite pitch, such as a submarine bell, a struck rod or a submarine oscillator. While our invention is of value in the reproduction of sounds from any tuned source it has unusual merit in the detection and reproduction of sounds from a source producing highly damped waves such as are created when a resonant body'is struck under water.

When the resonant body such as a submarine bell is struck under water, the vibrations of the bell, which would continue for some time were the bell in an easily comp'ressible medium such as air, are rapidl damped out so that probably only the first ew vibrations have suicient energy to be audible at any considerable distance from the, bell. Similarly the sound receiving surface or diaphragm of the hydrophone is very highly damped by the pressure u on it of the water in which it is submerge and as the diaphragm carries or is connected to the current-varyingdevice, such damping tends to reduce the desired variations in the strength of the current.

Heretofore hydrophone diaphragms have been usually constructed of thin metal, probably with the expectation that such a diaphra would respond most freely to the soun waves of small energy which reach it. In such case, however, the diaphragm, because of its lack of body, is damped very materially by the water pressure so that it may be said only to be moved by each sound wave having sutcient energy to move it and to have but little, if any, free vibration of its own.

Attem ts have been made to increase the 1921.' Serial N0. 470,115.

upon rods or tuning forks, but all such methods require that the tuning member shall be mounted upon a diaphragm the surface of which is in contact with the water. If this diaphragm is thin, the weight of the tuning member and microphone button is necessarily such that excessive damping occurs and th'e energy of succeeding sound waves is insufficient to build up the movements of the tuned member. If the diaphragm is large, such as the side of a ship, the energy is insufficient to build up to any appreciable extent the movements of the tuned member with its associated 'microphone button. Up to the present time none of these devices has been found practically useful.'

The object of our invention is so to construct a hydrophone that its diaphragm will respond with the least possible resistance to the sound waves which first reach it, and, because of its mass andresulting momentum, succeeding waves, even thou h of relatively small energy, will act to maintain it in motion notwithstanding the damping effect of the water with which it is in contact. The

Aotherparts of our hydrophone are so desource of submarine sounds, but this tuning is dependent upon the diameter, material and thickness of the diaphragm. We have therefore made the diaphragm of a suitable material and given it a thickness greater than has been usual in hydro hones hitherto made, so as to reduce the e ect of damping by water pressure which occurs when a thin 'diaphragm is used. We have found that for the usual submergence a diaphragm of phosphor bronze, about two tenths of an inch in thickness, will meet this requirement. To obtain the necessary frequenc of free vibration at the desired pitch of a out 12 15 per second we have used successfully a dlaemployed in hydrophones, utilize phragrn about four inches in diameter. An instrument thus proportioned responds to the first wave from the source striking it and the succeeding waves tend to prolong the vibrations of the diaphragm. The hydrophones which we have made using the above principles reproduce the clear ringing tone of the source and give a volume of reproduced tone far greater than has ever been obtained with the thin diaphragms hitherto' employed.

A second feature of our hydrophone has added greatly to its eiciency when used on shipboard at sea where the hydrophone is subjected to the noises produced by the ships machineryand by the water which impinges upon the side of the ship near which the hydrophone is 'mounted This second feature consists in the special form which we have given to the current-varying means or microphone button which is mounted upon the diaphragm lof the hydrophone. The microphone buttons, heretofore as the supports of the' front electrodes highly elastic disks such as mica or spring metal. This arrangement results in giving to the button a pitch of its own which is high, owing to the usual dimensions given to its several parts. Moreover, owing to the small inertia of the moving parts 'of the button, the moving parts are easily set-in motion and disturbing noises are produced frequently of sufficient intensity to overpower the note from the source which it is desired to hear. This diiculty is particularly great when the pitch of the button approximates that of the source. As most microphone buttons have been made to be mounted upon dia phragms, having small resonance when submerged, it has been usual to tune the buttons in order to increase their resonance to the sound waves from the source to the pitcher the source. rThe result has been that when disturbing noises were great, it was frequently impossible to distinguish the notes of the source.

In our hydrophone we have tuned the diaphragm and made it large and have used a button which is aperiodic or has a low natural pitch, a pitch far below that of the source. Tn fact, we have endeavored to make a button which has as little natural pitch of its own (i. e. is as aperiodic) as possible. This we have accomplished by mounting the front electrode of the button upon a diaphragm of soft rubber and have sub jected the rubber diaphragm to the least possible strain.

We have shown in the drawings the preferred form of device embodying our invention. y

Figure 1 is a front elevation of our hydrophone;

Fig. 2 being a vertical section thereof.

Fig. 3 shows in section the separated elements of the microphone in assembled re` lation, and

Fig. 4.- is a section of the assembled microphone.

1 is a heavy metal diaphragm made of elastic material which is tuned so as to have the greatest freedom of response, when submerged, for sound Waves of the frequency of the source. This diaphragm is clamped around its periphery to case 5 by clamping ring 2 which is secured to case 5 by a large number of screws, one of which is shown at 6. The casing 5 has an opening at the back through which the microphone 'may be inserted and which is closed by the screw plug case at the diaphragm a ring of packing' 4 is used and this4 packing ring is compressed by pressure ring 3. A stud 8 of insulating material having therein a threaded socket is screwed into the cent-re of the inner face of diaphragm 1 and acts as the support of button 7 Button 7 has an insulating washer 15 in which are slots which form pockets for the granular carbon 151 used as thc current-varying medium. These slots must have their longer dimension horizontal when the button is in position and the hydrophone is supported from its stem or its cable outside the case.` To permit the greatest ease in mounting the button properly upon the diaphragm and to eliminate all lost motion between the button and the diaphragm, a locknut 9 is provided. In assembling the button in the hydrophone, the shank of front electrode stud 19- (Fig. 3) is screwed as far as possible into the socketl in stud 8 and still have the longer dimension of the granular carbon pockets horizontal. .Locking nut 9, which had previously been screwed upon electrode stud 19, is then turned up agalnst stud 8 and thus firmly holds the button upon the centre of diaphragm 1. 10 and 11 are wires which connect the front and back electrodes of the button with the Wires leading through the watertight stem of case 5. v

1S is the cup of the button which is made of metal `and has soldered to its bottom by solder 131 back electrodo A14'ot highly pol ished carbonl or other conducting material, which does not readily oxidize. It has holes 132 to equalize the pressurewithin and Without the microphone. A washer 15 of felt or any other elastic non-conducting ma terial is inserted in the cup and fills with the least possible compression the space between back electrode 14 and front electrode 1S. `Washer 15 is shown ashaving holesl which, when the washer is in place between 14 and V18, form two parallel cavities in which the granular carbon 151 is placed. When assembled in the button, the Washer 15 is so placed that the holes have their greatest dimension horizontal. Front electrodelS has a highly polished-surface where it bears upon the granular carbon. Electrode 18 has been made ot' carbon in the buttons which We have manufactured but any conductor which can be highly polished and will not corrode may be used. Front electrode 18 is soldered to electrode stud 19 which has a threaded shank. The front electrode stud 19 is passed through a hole in the bottom of soft rubber cup 20. The bottom of rubber cup 20 thus forms a diaphragm for supporting the front electrode. Clamping washer 22 is next placed upon the shank of elect-rodestud 19 and locking nut 23 holds the clamping washer against the bottom of the rubber cup. lVhen the button cup has received the felt washer and the holes in the felt washer have been filled with the required amount of granular carbon, the rubber cup with theV front electrode is passed over the button cup and the sides of the button cup and the sides of the rubber cup are bound together by a whipping 2l of wire, thread or similar material. We have found that, if the pressure with which the front electrode stud is clamped upon the bottom of the rubber cup and the inside dimensions of the rubber cup and the outside dimensions of the button cu are correct, we can make a button which 1n itself has almost no natural frequency and that such frequency as inevitablyv must exist is far below that of the source of sound.-

The use of the word aperiodic has been applied above to a microphone button hav.-

ng a low n atural pitch, a pitch far below that of the source which is -to be received. Broadly stated, however, the wor'd aperiodic is not limited to a low natural pitch below that of the source to be received but is to be understood rather'in the following broad interpretation. Itis well known that most diaphragms or sound responsive devices respond more efiiciently to sounds of certain pitches than of other pitches. The range of frequencies to which a receiving device such as a microphone may respond varies materially, depending upon the broadness of the tuning in the microphone or in the receiving device. If a microphone is highly resonant at the same-frequency as the sound to be received the microphone is said to be highly tuned. By an aperiodic microphone we mean amicrophone which is not particularly resonant; that is, has a broad resonance curve and in addition to this the resonant frequency lies far outside of the range in which it is desired to operate the microphone for sound reception. TheA resonant frequency of an aperiodic microphone or any sound responsive device may be far below the pitch of the source which it is desired to receive or may, on the other hand, be far above the pitch of such a source. If, over the'range of reception in such cases, there is no resonance of the microphone, and the microphone responds as Well to one frequency in the range of desired frequencies as to another, and no better, the microphone is said to be aperiodic. In the present case, While it is preferable in some instances to use a microphone which has a resonance below the frequency of the source which is to be received, nevertheless it is possible to use a microphone which has a resonance far beyond the resonance of the source to be received.

In the operation of the device described above an aperiodic microphone is combined i with a highly resonant diaphragm. Not only is the mass of the microphone small as compared with that of the diaphragm but also the natural frequency of the microphone is far different from that of the diaphragm. Under such conditions the resonance curves of the two systems, in one case the diaphragm and in the other case the microphone, have their resonant frequencies widely separated. In coupling two such systems together the resonant frequencies remain practically unchanged and there results a high resonance point at the frequency of the diaphragm.y In this case, therefore, it is possible to' predict with great accuracy the frequency of the hydrophone', so that a hydrophone of a desired natural frequency may be easily obtained. In the case where the reson-ant frequencies of the individual systems lie somewhat close together the coulplingof the two effects a shifting apart of t resonant frequencies, producing a resonance curve of the combined systems which may be fairly broad and not so sharply tuned to an individual frequency. There is, therefore, a distinct advantage in combining systems such as described above for not only is a highly tuned hydrophone obtainable v in this way but also it is fairly simple to obtain the desired resonant frequency.-

What we claim as our invention is 1. A hydrophone comprising a tuned sound-receiving diaphragm of considera.- ble thickness having a microphone button located thereon, said microphone being aperiodic.

2. A hydrophone comprising a thick highly tuned diaphragm and a microphone button mounted thereon and comprising a. diaphragm of liexible material under substantially no tension and an electrode attached to said flexible diaphragm.

3. A hydrophone comprising a relatively thick highly tuned diaphragm and la microphone button comprising a metallic cup having an electrode mounted therein and a rubber cup having its sides enclosing entirely the sides of said metallic` cup, a second electrode mounted within said rubber cup, said cups forming a chamber and resistance varying material within said chamber.

4. A hydrophone comprising a diaphragm mi and a microphone button mounted thereon,

to respond yfreely to sounds of a much lower pitch. I

5. In a hydrophone, -a tuned diaphragm and a microphone button attached thereto to vibrate therewith, said microphone button comprisin a diaphragm of unstrained soft rubber an having one of its electrodes attached thereto whereby the period of vibration of said microphone Will depend upon the period of vibration of said tuned vdiaphragm.

6. A hydrophone comprising Ia highly resonant diaphragm and an aperodic microphone button located thereon to be operated thereby.

1 WALTER H. BARBER.

RICHARD D. FAY. 

